This invention relates generally to perceiving rotation of an object. More specifically, the invention relates to the devices used for making such perceptions. In greater specificity the invention relates to an optical technique for detecting rotation or angular displacement of an object by utilizing the technology known as micro-electro-mechanical systems or xe2x80x9cMEMSxe2x80x9d.
Typical MEMS-based gyroscopes use capacitive pick-offs to detect change in angular acceleration/rotation. This technique, however, is limited in sensitivity. This is particularly true as dimensions of the device are decreased, as capacitance varies linearly with area. One technique to increase the capacitance and thus sensitivity of these designs is to decrease the distance between the two parallel plates that form the capacitor. Decreasing this dimension presents its own problems, as variations in thickness and spacing across the surface of the parallel plates then play a much greater role in other performance shortcomings. Larger parallel plate spacings, on the order of a few microns, are therefore generally utilized to help increase repeatability and percent uniformity. Capacitive techniques result in a passive means of detection, meaning that additional low noise amplification and filtering circuitry must be employed to extract accurate rotational rates. Piezo-electric techniques cannot provide nearly enough sensitivity for many desired applications. Because of low steady-state signal levels, high sensitivity MEMS-based gyroscopes are difficult to realize.
To gain the full potential of MEMS based gyroscopes, significant improvement in sensitivity over the prior-based methods must be made.
Traditional gyroscope design incorporates a rapidly spinning element. The invention does not use a rapidly spinning component but instead uses a structure that allows the sensing of rotational movement and position of one structure with respect to another. The invention is based upon the integration of an optical resonant cavity and a photodiode. This combination is used to detect minute perturbations due to angular acceleration, such as those that may be generated by the Coriolis Force, perturbations due to constant angular acceleration otherwise known as rotational velocity, and angular displacement of one object with respect to another.
For example, a Fabry-Perot cavity, consisting of two parallel semitransparent mirrors, can be used in conjunction with a light source. One of the two mirrors is fixed in position while the other is allowed to rotate with respect to the first mirror around a fixed rotational axis. A resonant cavity is thereby formed on either side of the axis, though only a single upper and lower mirror are used. If monochromatic light is used to illuminate the upper mirror of the cavity and the distance between the upper and lower mirrors is an integral multiple of half wavelengths of this light, then a resonant condition will exist and the transmission of light through the mirrors will be optimized.
As the upper mirror is rotated, the distance between the two mirrors will become altered. One resonant cavity will xe2x80x9cseexe2x80x9d a decrease in cavity length while the other will see an equal but opposite increase in cavity length. As the distance between the two mirrors is changed, the light transmitting the mirrors will also be changed. Photodiodes integrated on either side of the torsional/rotational axis are used to sense the change in distance as a change in photo-generated current. By monitoring the change in photocurrent, the amount of change in rotation can be calculated. The photo-currents collected from the two cavities can be differentially amplified to further the sensitivity of the device.
Other objects, advantages and new features of the invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanied drawings.